1. Field of the Invention
The present invention relates to ultrasonic monitors for measuring heart rates and pulse rates in living subjects.
2. Description of the Related Art
Measuring heart and pulse rates in living subjects has become a valuable tool during physical exercise and for health monitoring. The heart rate and pulse rate of a subject are related. Heart rate may be defined as the number of heart contractions over a specific time period, usually defined in beats per minute. A pulse is defined as the rhythmical dilation of a vessel produced by the increased volume of blood forced through the vessel by the contraction of the heart. Since heart contractions normally produce a volume of blood that can be measured as a pulse, heart rate and pulse rate are ideally the same. However, a pulse rate may differ from the heart rate during irregular heart beats or premature heart beats. In this case, a heart contraction may not force enough blood through a blood vessel to be measured as a pulse.
A pulse rate is measured by counting the rate of pulsation of a subject's artery. The heart rate is measured by sensing the electrical activity of the heart based on electrocardiograms (for example EKG or ECG). Individuals who want to increase their endurance or performance may wish to exercise while maintaining target heart rates. Conversely, subjects with a history of heart disease or other heart related condition should avoid exceeding a certain heart or pulse rate to reduce unnecessary strain on their heart.
Most subjects that require continuous heart rate readings choose a monitor that requires a chest strap. Though they provide heart rates continuously, chest straps are cumbersome and generally undesirable to wear. In addition to chest strap solutions, portable patient monitors (e.g., vital signs monitors, fetal monitors) can perform measuring functions on subjects such as arrhythmia analysis, drug dose calculation, ECG waveforms cascades, and others. However, such monitors are usually fairly large and are attached to the subject through uncomfortable wires.
Pulse rate can be measured at the wrist. The shallow depth of the radial artery in the wrist offers a number of advantages for achieving continuous pulse detection at the wrist. Prior sensors that monitor pressure pulses in the wrist have not been effective. Pressure pulses are attenuated by the tissues between the artery and the sensor. Most of the high frequency signal components are lost because of the attenuation. Additionally, muscle movement may create substantial noise at the pressure sensors. The low frequency noise signals make it very difficult to reliably identify low frequency blood pressure pulses.
Ultrasonic monitors using sonar technology were developed to overcome noise signal problems. Ultrasonic monitors transmit ultrasonic energy as a pulse signal. When a power source drives a transducer element, such as a piezoelectric crystal, to generate the pulse signal, the ultrasonic pulse signal is generated in all directions, including the direction of the object to be measured such as a blood vessel. The portion of the ultrasonic pulse signal reaching the vessel is then reflected by the vessel. When the blood vessel experiences movement, such as an expansion due to blood flow from a heart contraction, the reflected pulse signal experiences a frequency shift, also known as the Doppler shift.
When either the source of an ultrasonic signal or the observer of the sonar signal is in motion, an apparent shift in frequency will result. This is known as the Doppler effect. If R is the distance from the ultrasonic monitor to the blood vessel, the total number of wavelengths λ contained in the two-way path between the ultrasonic monitor and the target is 2R/λ. The distance R and the wavelength λ are assumed to be measured in the same units. Since one wavelength corresponds to an angular excursion of 2π radians, the total angular excursion φ made by the ultrasound wave during its transit to and from the blood vessel is 4πR/λ radians. When the blood vessel experiences movement, R and the phase φ are continually changing. A change in φ with respect to time is equal to a frequency. This is the Doppler angular frequency Wd, given by
      W    d    =            2      ⁢      π      ⁢                          ⁢              f        d              =                            ⅆ                                          ⁢          Φ                          ⅆ          t                    =                                                  4              ⁢              π                        λ                    ⁢                                    ⅆ              R                                      ⅆ              t                                      =                              4            ⁢            π            ⁢                                                  ⁢                          V              r                                λ                    where fd is the Doppler frequency shift and Vr is the relative (or radial) velocity of target with respect to the ultrasonic monitor.
The amount of the frequency shift is thus related to the speed of the moving object from which the signal reflects. Thus, for heart rate monitor applications, the flow rate or flow velocity of blood through a blood vessel is related to the amount of Doppler shift in the reflected signal.
A piezoelectric crystal may be used both as the power generator and the signal detector. In this case, the ultrasonic energy is emitted in a pulsed mode. The reflected signal is then received by the same crystal after the output power source is turned off. The time required to receive the reflected signal depends upon the distance between the source and the object. Using a single crystal to measure heart rates requires high speed power switching due to the short distance between source and object. In addition, muscle movement generates reflections that compromise the signal-to-noise-ratio in the system. The muscle movement noise has a frequency range similar to the frequency shift detected from blood vessel wall motion. Therefore, it is very difficult to determine heart rates with this method. The advantage of this approach, however, is low cost and low power consumption.
In some ultrasonic signal systems, two piezoelectric elements are used to continuously measure a pulse. The two elements can be positioned on a base plate at an angle to the direction of the blood. In continuous pulse rate measurement, the Doppler shift due to blood flow has a higher frequency than the shifts due to muscle artifacts or tissue movement. Therefore, even if the muscle motion induced signals have larger amplitudes, they can be removed by a high pass filter to retain the higher frequency blood flow signals. The disadvantages of continuous mode over pulsed mode are higher cost and more power consumption
Several wrist mounted ultrasonic monitor devices are known in the art. However, ultrasonic signals are prone to diffraction and attenuation at the interface of two media of different densities. Thus, air in the media or between the monitor and the subject's skin make ultrasonic energy transmission unreliable. Prior ultrasonic monitors require applying water or an aqueous gel between the transducer module and the living subject to eliminate any air gap. Because water and aqueous gels both evaporate quickly in open air, they are not practical solutions.
U.S. Pat. No. 6,843,771 disclosed the use of thermoplastic and thermoset gels as the transmission medium for ultrasonic signals to overcome the problems associated with water and aqueous gel solutions. In U.S. Pat. No. 6,716,169, Muramatsu et al. disclosed a soft contact layer based on silicone gel, a type of thermoset gel, as the medium for the ultrasonic signal transmission. These gels mainly consist of a large quantity of non-evaporating (at ambient condition) liquid diluents entrapped in a lightly cross-linked elastomeric network. These cross-linked networks can be either physical in nature, such as in the thermoplastic gels, or chemical in nature, such as the thermoset gels.
Synthetic thermoset and thermoplastic gels have disadvantages. The liquid diluents, though entrapped in the elastomeric network, can still diffuse into the skin of a user upon contact over a period of time. Since silicone gels use silicone oil as diluents, diffusion of silicone oil is an important health concern, Diffusion of these oils into body tissues can cause biological problems. Synthetic thermoset and thermoplastic gels also tend to be soft gels. Though a softer gel allows better contact with the skin and results in better ultrasonic transmission, soft gels are weak, difficult to handle and difficult to attach to ultrasonic transmitters.
Efficiency of the transmitting transducer is an important feature in wrist worn and other small heart rate monitors. Transmission of an ultrasonic signal by a transmitting transducer can be made more efficient by use of a reflector. Transmission signals generated away from target can be reflected using a reflector on one or more sides of the transducer. Some heart rate monitors include a foam substance having air voids underneath the piezoelectric crystals. As illustrated in FIG. 1, a foam layer 120 may be placed within ultrasonic module 110 underneath transducers 130 and 140. The foam material air voids partially inhibit ultrasound energy penetration and provide fairly effective reflection of ultrasound signals. With this foam backing, some of the ultrasonic signals directed towards the foam are reflected toward the desired direction. The disadvantage to incorporating foam layers is that they are manually installed during manufacture. Other prior systems increase efficiency by separating the two piezoelectric crystals by a channel on a base plate. This reduces crosstalk between the transducers to some degree but does not eliminate the loading or dampening effect caused by the base plate.
Heart rate monitors that provide continuous heart rate readings through a transmission media are useful. The transmission media should be biocompatible and not dry out during the monitoring, leave an uncomfortable wet film, or be difficult to generate and apply.